Low cloud and rain has hindered Australian efforts to find debris that may be linked to the missing Malaysia Airlines flight, with the captain of the first plane to return from the search zone describing weather conditions as "extremely bad".

The Australian Maritime Safety Authority (AMSA) is coordinating efforts to identify two objects seen on commercial satellite images in a remote area 2,500 kilometres south-west of Perth.

The large objects, which were spotted by satellite five days ago, are the most promising find in days as searchers scour a vast area for flight MH370, which is lost with 239 people on board, including six Australians.

An RAAF Orion plane was unable to locate the debris overnight because of limited visibility from cloud and rain.

"The weather conditions were such that we were unable to see for very much of the flight today but the other aircraft that are searching, they may have better conditions," Flight Lieutenant Chris Birrer told reporters.

A reporter aboard a US Navy aircraft told America's ABC network that their sweeps of the area yesterday only turned up a freighter and several pods of dolphins.

"This is a plane that gets very close to the water, most of the time we were skimming at about 300 feet (about 90 metres) above the water - you could see a basketball if it were in the water," he said.

"Plus they had high-tech radar, sweeping 16 miles (about 25km) on each side of the plane. If there was something there, this plane surely would have spotted it."

Prime Minister Tony Abbott has emphasised the remoteness of the search location.

"We don't know what that satellite saw until we can get a much better, much closer look at it, but this is the first tangible break-through in what up until now has been an utterly baffling mystery," he said as he arrived in Papua New Guinea for talks on trade, the economy and Manus Island.

"We are throwing all the resources we can at it. We will do everything we humanly can to try to get to the bottom of this."

Australian search planes will continue the hunt for the debris today, along with aircraft from the US and New Zealand.

Planes involved in southern search corridor

Country

Planes

Australia

5

Indonesia

4

Japan

4

China

3

Malaysia

2

India

2

South Korea

2

New Zealand

1

US

1

UAE

1

Information from Malaysia's transport minister.Current as of Friday morning.

A Norwegian ship arrived in the 23,000-square-km search zone last night after responding to an Australian request to re-route from its original path to Melbourne.

Another merchant ship is travelling to the area, one of six involved in the search since a shipping broadcast was issued on Monday night.

HMAS Success is also en route, but AMSA says the Durance class ship will not reach the zone for "some days".

Defence Minister David Johnston says it could be several days before people have any answers about whether the debris is linked to the missing flight.

The area where the objects were spotted roughly corresponds to the far end of a southern track that investigators calculated the aircraft could have taken after it was diverted.

Transport Minister Warren Truss says more satellite images are being taken of the area.

"That work will continue, trying to get more pictures and stronger resolution so that we can be more confident about where these items are, how far they've moved and therefore what efforts should be put into this search effort," he told AM.

Mr Truss says authorities have dropped buoys in the search location to analyse the tide and wind to pinpoint where the objects may have drifted since the images were taken.

"Clearly they will have moved since March 16 when those images were originally captured," he said.

"They will have moved because of tides and wind and the like so the search area is quite broad."

The satellite image company that provided the images, DigitalGlobe, said the sheer number of images covering a large search area contributed to a delay in authorities identifying what they believe is debris.

Spokesman Turner Brinton says the company's five high-resolution satellites capture more than 3 million square kilometres of Earth imagery each day.

"This volume of imagery is far too vast to search through in real time without an idea of where to look," he said.

Tracking flight MH370

University of Western Australia Professor of Oceanography Charitha Pattiaratchi says based on currents in the area, if the debris is from the plane it probably would have entered the water around 300 to 400 kilometres to the west.

He says the search area covers an ocean ridge known as Naturalist Plateau, a large sea shelf about 3,500 metres deep. The plateau is about 250 km wide by 400 km long, and the area around it is close to 5,000 metres deep.

"Whichever way you go, it's deep," Professor Pattiaratchi said.

Flight MH370 has been missing since it disappeared en route to Beijing from Malaysia on March 8.

So far the investigation has focused on the possibility that the plane was deliberately diverted from its flight path.

The plane is thought to have travelled in either of two directions: north-west into Asia or south-west into the Indian Ocean.

Australia has been leading the search in the southern vector, specifically an area 3,000 kilometres south-west of Perth.

AMSA says the search zone covers 600,000 square kilometres of ocean and has been plotted using data based on the last satellite relay signals sent by the plane.

The search now encompasses an area stretching 7.7 million square kilometres - an area larger than the entire land mass of Australia.

Search for wreckage on ocean floor 'could take years'

US oceanographer Dr Dave Gallo, who coordinated the search for the Air France plane that crashed in the Atlantic Ocean in 2009, told 7.30 that any recovery could take years.

Dr Gallo says the search area in the southern Indian Ocean could be five or six times larger than the search zone for the Air France wreckage.

"Once you get the X-marks the spot from the floating debris or from the last-known position, then you want to know how wide should we look and in this case it's going to be difficult," he said.

"In the case of Air France 447, we had some ACARS data, some bursts of information that abruptly stopped after 12 minutes, so we knew the search circle was roughly 80 miles in diameter - that's a big area.

"In this case we don't have that; we're going to have to make some assumptions. It may be a large search area to begin with, but it's a start."

Dr Gallo says there are two methods that could be used to search for the wreckage once an area of interest is located.

"One is what we used on Air France 447 and the Titanic most recently. They're called AUVs - autonomous underwater vehicles - and they look like torpedoes," he said.

"They carry sonars and cameras and we launch them from the back of the boat and down they go by themselves and they run very, very crisp parallel lines, just like the best farmer's field ploughing back and forth.

"And then they come back to the ship, we upload the data, see what they saw and recharge them, then back under water.

"The more traditional way is to tow a system of long cable behind the ship and with the surface ship going back and forth over the sea floor.

"Maybe in this case we need both of those."

'My son is still alive'

The relatives of the 153 Chinese passengers - about two-thirds of those people on flight MH370 - are refusing to give up hope despite the announcement that debris may have been found.

Wen Wancheng, 63, from China's Shandong province, says he will not giving up hope that his son is still alive.

"Do you think I would believe my son is gone?" he said.

"Can I believe he is in the sea? My son is still alive. My son is still alive ... I don't believe the news."

Meanwhile, Malaysian authorities have launched an investigation after anguished Chinese relatives of passengers on the missing flight stormed into a media centre in Kuala Lumpur on Wednesday night, calling on authorities to "give us back our families".